1. Field of Invention
This invention is directed to a method and an apparatus of obtaining fast sensitive miniature magnetic sensors which measure local fields for recording head applications, as well as for other applications requiring remote sensing of weak magnetic signals.
2. Description of Prior Art
The fluxgate magnetic field sensor continues to be the preferred transducer for magnetic field measurements, not only because the supporting electronics is fairly simple and reliable, but also because new developments in materials sciences continue to push the noise figure to its intrinsic quantum limit. It is now approaching the point where the fluxgate device becomes an attractive alternative to the more complicated and costly SQUID fluxmeter. A fluxgate magnetometer is a device which measures magnetic fields utilizing the nonlinear magnetic characteristics of the ferromagnetic core material. The traditional ferromagnetic core material contained in the art consists of sheets or ribbons of permalloy, or amorphous ribbons with typical composition of iron-boron. The resolution of a fluxgate device is limited by Barkhausen noise associated with domain-wall motion in the core material. The sensitivity of fluxgate magnetometers using magnetic metal ribbons is in the order of 17 pT at room temperatures. The sensitivity of a SQUID device is about 1 pT at liquid helium temperatures (please refer to O. V. Nielsen, B. Herhando, J. R. Tetersen, and R. Primdahl, xe2x80x9cMiniaturization of low cost metallic glass fluxgate sensorsxe2x80x9d, J. Mag. Mag. Matr., 83, 405, 1990, for a general background on fluxgate magnetic sensors).
Furthermore, a fluxgate sensor contained in the art measures the global field which appears over the whole volume of the sensor core as an average. There is a need for sensitive fluxgate probes which measure the local fields associated with magnetic defect sites in materials for nondestructive evaluations, for example. Other applications include dynamic measurements of the local fields at high speeds associated with the data bits stored in a magnetic medium. The present fluxgate devices contained in the art have only been used for static or quasi-static field measurements. The possibility that a fluxgate sensor can measure a weak digital magnetic signal appearing in a data string at a high flow rate has never been considered in the art.
A magnetic induction-type recording head contained in the art measures the induction currents whenever the bits change signs in a data string. A more efficient way to measure a string of digital data is to measure the bits themselves, rather than their sign changes. For example, two consecutive bits of the same sign can not be measured in a straightforward manner by using an induction-type recoding head. A fluxgate recording head thus provides more efficiency, especially when synchronization is applied to the reading action. Synchronization locks the measurements at a specific frequency concurrent with the flow of the data string at fixed phases, which can reduce the noise content significantly, and hence improve the signal-to-noise ratio.
Although a fluxgate sensor has already been shown to provide the highest sensitivity in measuring a magnetic field at room temperatures, it has not yet been used as a magnetic recording head device reading data stored in a magnetic medium flowing at high speed. A ring-core fluxgate magnetometer contained in the art occupies a considerable volume. The ring-shaped geometry of the sensor core wound by inductor coils precludes the possibility that the sensor core can be configured into an open-arm structure detecting a magnetic field at a remote site. The fabrication of a ring-core fluxgate device is incompatible with the current printing-circuit technologies. Today, the biggest market for magnetic sensors is called for by the magnetic recording head industries, and there exists a constant need for miniature sensitive magnetometers included with recording head devices whose fabrication is compatible with the printing-circuit technologies facilitating mass production in large volumes, not only to increase reliability, but also to reduce costs.
Accordingly, it is an objection of the invention to address one or more of the foregoing disadvantages or drawbacks of the prior art, and to provide such improved method and apparatus to obtain sensitive miniature fluxgate devices for the measurement of weak magnetic fields.
Other objects will be apparent to one of ordinary skill, in light of the following disclosure, including the claims.
In one aspect, the invention provides a method of achieving fluxgate operation allowing for detection arms to be extended from the sensor core so as to measure a local magnetic field at a remote site. The two pickup coils may be connected providing the same sense of induction or opposite sense of induction. The sensor geometry allows the sensor circuit to be miniaturized whose fabrication is compatible with the current printing circuit technologies.
In another aspect, the invention provides an apparatus of performing dynamic fluxgate operation measuring a weak magnetic signal associated with a data string. The excitation coil is synchronized with the data flow locked at specific phases so as to enhance signal-to-noise ratio, and hence to improve sensitivity.